Compact, rapidly rotating astronomical objects (like neutron stars) are often observed to emit streams of high energy particle radiation along their poles. Near supermassive black holes in galactic cores, the velocity of these jets can reach 0.99c.

Could such a jet be used to accelerate an intergalactic probe to relativistic speeds?

Of course, this question is about fictional aliens living close to such a jet, since our galaxy does not have it.

$\begingroup$I've removed the secondary questions about the ship and electronics, please feel free to ask them as separate questions but to work well questions here should each focus on one topic.$\endgroup$
– Tim BFeb 1 '17 at 12:32

$\begingroup$Asking multiple questions in one is not recommended as it makes it hard to answer with a concise and focused reply. Additionally it becomes much harder to rate answers as to whether one is better than another as the "best" answer to each part of your question may be held in different answers. See tips on how to fix the problem.$\endgroup$
– Tim BFeb 1 '17 at 12:32

$\begingroup$@Tim B I accept what you said about the antimatter question. But by this question, the questions are very closely related: The main problem potentially making the idea impractical is, that the jet may destroys the vessel.$\endgroup$
– b.LorenzFeb 1 '17 at 19:11

7 Answers
7

This answer will briefly review the properties and characteristics of atsrophysical jets relevant to accelerating space craft to relativistic velocities. Particularly with relevance to an astrophysical jet generated by a supermassive black hole. This is a suitable type example for this scenario. It will also review the state of art of this propulsion mechanism.

Astrophysical jets are physical conduits along which mass, momentum,
energy and magnetic flux are channeled from stellar, galactic and
extragalactic objects to the outer medium. Geometrically, these jets
are narrow (small opening angle) conical or
cylindrical/semi-cylindrical protrusions (e.g., Das 1999). Jets are an
ubiquitous phenomenon in the universe. They span a large range of
luminosity and degree of collimation, from the most powerful examples
observed to emerge from the nuclei of active galaxies (or AGNs) to the
jets associated to low-mass young stellar objects (YSOs) within our
own Galaxy. In the intermediate scales between these two extremes one
finds evidences of outflows associated to neutron stars, massive X-ray
binary systems (with SS433 being the best example of this class),
symbiotic stars, and galactic stellar mass black holes (or
microquasars).

As largely stressed in the literature, most of these outflows, despite
their different physical scales and power, are morphologically very
similar, suggesting a common physical origin (see below). For example,
in one extreme, AGN jets have typical sizes ³ 10^6 pc 1, nuclear
velocities ~ c (where c is the light speed), and parent sources (which
are massive black holes) with masses 10^6-9 M􀀀 2 and luminosities
~10^43-48 L􀀀 3;

The scale of astrophysical jets ranges from one parsec (pc) through kiloparsecs to one million pc. For example, this anticipates accelerating spacecraft to reativistic velocities. If a jetrider spacecraft was acceelrated to relativistic velocity by travelling along a one million parsec astrophysical jet, this is a distance of approximately three million light years, it only needs to accelerate at an average acceleration of fifty (50) metres per second per annum. This acceleration phase will take six million years. Any technological civilization carrying out projects of this kind will operating on extremely long timescales and over extremely long distances.

There are major questions about the stability and variability of astrophysical jets. The environment inside an astrophysical jet is one of the most hostile known. The long-term survival of any spacecraft could only be accomplished by extremely advanced technologically societies.

The technical literature doesn't readily yield any useful estimates for momentum transfer necessary to accelerate a spacecraft. It is recommended that if the OP wishes to obtain this information that one way to do so is by contacting the authors of scientific papers on astrophysical jets and asking them what is the estimated average momentum transfer. Since there is a nuber of different types of astrophysical jets the OP should select the type of astrophysical jet needed for the worldbuilding and make inquiries. Most scientists are delighted when someon shows an interest in their work. Even if it is some thing as quirky as translating their research into a concept like spacecraft propulsion. The OP should try to obtain information about momentum transfer per unit area. This can be translated into acceleration based on the mass of the jetrider probe.

This answer suggested this question might be premature. This is a brief note by Yoshinari Minami that he is currently undertaking research into the possibility of spacecraft using astrophysical jets as propulsion. To date, no results have been published.

A Journey to the Stars: Space Propulsion Brought About by Astrophysical Phenomena Such as Accretion Disk and Astrophysical Jet

Yoshinari Minami

Space Propulsion Brought About by Astrophysical Phenomena Here,
astrophysical phenomena refer mainly accretion disk and astrophysical
jet around black holes. Accretion disk is rotating gaseous disk with
accretion flow, which form around gravitating object, such as white
dwarfs, neutron stars, and black holes. At the present day, owing to
the development of observational technology, it is believed that
accretion disk causes the various active phenomena in the universe:
star formation, high energy radiation, astrophysical jet, and so on.

It should be noted; these stars such as white dwarfs, neutron stars,
and black holes have a strong magnetic field (108 Tesla－1011 Tesla).
Matter falling onto an accretion disk around black hole is ejected in
narrow jet moving at close to the speed of light like an accelerator.
Entity of the astrophysical jet is a jet of plasma gas from the active
galactic nucleus (accretion disk in there). It is said that such
astrophysical jet is held together by strong magnetic field tendrils,
while the jet's light is created by particles revolving around these
wispthin magnetic field lines. Furthermore, since the system of black
hole and accretion disk is like a gravitational power plants, the
energy of the heat and the light are produced by the release of
gravitational energy.

Although the system of accretion disk and astrophysical jet around
black holes are currently left many unresolved issues, the elucidation
of these mechanisms and principles that are common to the entire
universe may provide a new space propulsion principle. Especially, the
breaking of magnetic field lines and magnetic field reconnection are
possible to produce many kinds of charged particles such as electron
positron pairs. Generally, in a high-temperature plasma, electron -
positron pairs are readily formed by collisions between the high
energy protons, electrons, photons. Since the dynamics of the
accretion disk has been decided by a magnetic field, it is important
that solving the dynamics of the magnetic field.

The application of mechanism of accretion disk and astrophysical jet
around black holes will lead to the concrete system design of
propulsion engine and power source installed in space drive propulsion
system [1,2,5]. Author is now investigating above-stated research in
detail.

A particle-beam pushed sail has many advantages as a propulsion system
for interstellar flight:
1. Light weight. Since the sail reflecting the beam is not a physical object, it can be made extremely light.
2. Large target. The actual reflecting area of a mini-magnetosphere sail is much larger than the magnetic loop itself. Therefore, the sail
can be quite large. This makes the aiming, beam stabilization, and
beam divergence problem much lower.
3. High acceleration. The limit on the acceleration of a lightsail is set by the temperature limit of the sail material (Landis, 1997,
2000). Since the active part of a mini-magnetosphere magnetic sail is,
in fact, a magnetic field, there is not a significant thermal limit.
The field is sustained by a plasma, which likewise is not subject to
melting. Only the physical magnet itself is thermally limited, and
this magnet is extremely small compared to the sail area. By
increasing the acceleration, cruise velocity can be achieved in a
shorter distance, again decreasing the requirements for beam
divergence and the aiming and stabilization difficulty.
4. Higher momentum/energy ratio. A particle beam has a much more momentum per unit energy than a photon (e.g., laser) beam, and hence
transfers force to a sail with better energy efficiency. The
(relativistically correct) relation between momentum (p) and energy
(E) for both particle beams and photon beams is:

p^2 = E^2/c^2 + moE (1)

(note that energy here refers only to the applied kinetic energy of
the beam, not including the rest-energy mc^2 of the particles). For a
photon beam, the rest mass mo is zero. It is clear that the ratio of
momentum to energy increases directly as the rest mass of the
particles composing the beam increases. Since the ratio of momentum to
energy equals force produced on the sail per unit of beam power, a
particle-beam pushed sail has a higher energy efficiency.

The proposed mechanism for particle-beam propulsion is the use of a mini-magnetosphere.

the invention of the mini-magnetosphere plasma propulsion, or "M2P2"
(Winglee et al. 2000, 2001), has brought the idea of a particle-beam
pushed sail closer to reality. The particle beam is reflected by a
magnetic field. In the mini-magnetosphere, the magnetic field is
inflated to large areas by the injection of a plasma, and hence large
magnetic field areas are possible with only a small physical
structure.

One deficiency of a mini-magnetosphere is that the plasma will slowly leak away. One way of replenishing this will be capturing some of the plasma in the astrophysical jet itself.

Many astrophysical jets themselves have velocities close that of lightspeed. This suggests the kinetic energy and momentum that can be transferred to an accelerating spacecraft will be high. This makes the possibility a jetrider vehicle attaining relativistic velocities is itself high. Any propulsion technology that advantage of this will need to be extremely robust.

This suggests that if the advanced technological society launching probes via astrophysical jets is sufficently advanced it construct a pusher plate made of nuclear dense materials. Effectively, a thin sheet of neutronium. This will certainly survive relativistic plasma, high-energy radiation, and even electron-positron plasma. This does assume extremely highly advanced technology, but the aliens didn't possess suitable technology they wouldn't attempt using astrophysical jetriding vehicles of any kind.

While this add considerable mass to any spacecraft if that probe is riding a one megaparsec astrophysical jet even low rates of acceleration will eventually result in relativistic velocity.

Many question of the issues the OP wanted to find answers are not readily available without more direct inquiry, research into space vehicles propelled by astrophysical jets is only beginning, and the level of technology required for viable spacecraft operating in the environment of astrophysical jets is so far beyond any conception of our current state of knowledge that it is simpler to assume the aliens with jetrider technology can do it without any explanation.

You're basically talking about an Electric Sail here. However, you need to swap the wires over on your handwavium engine as you're working from neutrons instead of the intended electrons.

The electric sail consists of a number of thin, long and conducting tethers which are kept in a high positive potential by an onboard electron gun. The positively charged tethers deflect solar wind protons, thus extracting momentum from them. Simultaneously they attract electrons from the solar wind plasma, producing an electron current. The electron gun compensates for the arriving electric current.

One way to deploy the tethers is to rotate the spacecraft, using centrifugal force to keep them stretched. By fine-tuning the potentials of individual tethers and thus the solar wind force individually, the spacecraft's attitude can be controlled.

E-sail missions can be launched at almost any time with only minor variations in travel time. By contrast, conventional slingshot missions must wait for the planets to reach a particular alignment.

You won't of course need an on-board electron gun, the star will do this for you...

Looking a bit like this

Or a backwards parachute. Obviously the "living/instrument" space will need some pretty good radiation shielding....

$\begingroup$Yes I am aware of the electric sail technology, the question is, whether the jet will provide sufficient thrust while not destroying the sail.$\endgroup$
– b.LorenzJan 30 '17 at 16:33

$\begingroup$Sure. Just like with any other sail, you decrease the surface area if the stream is strong enough to cause damage. As your speed and momentum increases, let the sails out more. Just the same as sailing on a lake in strong winds.$\endgroup$
– user10945Jan 30 '17 at 16:35

$\begingroup$I don't think it's purely from neutrons. We can see them because they radiate, and I have read, that they consist of electrons or maybe positrons too.$\endgroup$
– b.LorenzJan 30 '17 at 16:48

1

$\begingroup$@b.Lorenz I suggest Pete's answer is the same regardless of the particles involved. You find the particles that are most dominant and build a net that traps those particles. If all the particles are of use, build something that traps them all.$\endgroup$
– SRMFeb 5 '17 at 14:52

1

$\begingroup$@SRM As far as we know, it's quite impossible for effectively trap neutrinos with a 'sail' or 'net', while from light or electrons one can easily harvest momentum. So the composition of the stream does matter.$\endgroup$
– b.LorenzFeb 5 '17 at 16:53

Living close to one strikes me as not likely for any life form as this page explains.

But more practically there's the point that to get into these "jet streams" (for want of a better term) you'd need to be able to travel galactic distances in reasonable timescales already. And if you can do that, you already have a better propulsion system than a sail.

Keep in mind that "close" is relative. Even if I have to travel just 0.01% of the galactic bulge's diameter that's still 0.01% of 10,000 light years which is a whole light year ! And I'd need to be much further away to make the existence of any civilization likely.

$\begingroup$Surely, you have right. my scenario is impossible. But you have avoided my question: ''is it such a sail theoretically possible?''$\endgroup$
– b.LorenzJan 30 '17 at 18:56

$\begingroup$Impossible, no. Insanely impractical yes. The problem is that your "sail" and it's crew and instruments are in an environment that's just hostile to life in all ways. You'd need a lot ( a lot a lot ) of protection and at 0.99c anything can kill.$\endgroup$
– StephenGJan 30 '17 at 20:01

1

$\begingroup$Good. If you explain that the ship will be inevitably destroyed, I will accept your answer. Note, that I didn't mentioned living crew. Only a probe, hoping that it's possible to maintain some instruments intact.$\endgroup$
– b.LorenzJan 30 '17 at 20:04

$\begingroup$If the story requires, one can make up excuses why they have to rely on such a sail. Eg.: once they had fast space drive, but lost it due to a sociological crisis and have to reinvent it. Or their transportation system relies on wormholes, but by some reason they can't find or generate one in the desired direction and huge distance. Or they use some for of Bussard ramscoop, but where they live, the intergalactic medium is much less dense than the interstellar, so they have to search other means to leave the galactic plane.$\endgroup$
– b.LorenzJan 30 '17 at 20:09

This depends on what you mean by "intergalactic speeds". There is certainly a lot of energy there for you to work with, unfortunately there's also a LOT of energy there to do inconvenient things like obliterate your probe.

Assuming you managed to construct the probe appropriately then it could certainly ride the jet and get some energy out of it - however there is another problem.

The further you move on the jet the lower the acceleration becomes. The particles will spread out and slow down and the jet will become steadily weaker. You would need to get as much acceleration as possible as early as possible and then after that you are going to be coasting with whatever boost you managed to pick up and no more to add to it.

(This also doesn't address how you are going to slow down at your destination!).

$\begingroup$Though the probably greater problem (for any matter-based lifeforms), is that until you're up to cruising speed, you are also putting yourself in the path of some of the most energetic radiation we know of. What shield mechanism would keep safe from that?$\endgroup$
– CatalystFeb 4 '17 at 16:17

$\begingroup$@Catalyst A big subject all on its own.$\endgroup$
– Tim BFeb 4 '17 at 16:27

Answers so far imply travelers are being pushed by the jet, sailing along under its impetus. The way to use this one of these things is to straddle it and ride it Strangelove-style. With the jet behind you it will be safer and so much cooler. Don't forget the hat!

1: Find a neutron star that is not spinning so fast. Or if you have to use a spinning one, approach it on the axis and gently touch down with a ship mounted Lazy Susan, so you do not have to spin so much.

2: Turn off one jet. Hard to move if your rocket is shooting equally forwards and backwards. Turning off will be tricky and this aspect of the scheme is left as an exercise to the reader. Do it in a way you can turn it back on again; you will need it for #4.

Next:steering. Otherwise your flight path is going to be like a balloon with the air let out of it. I propose that a magnetic field generating widget ring could be clamped across the jet orifice. Magnetic fields don't get worn away or otherwise perturbed by a jet of energetic particles. By tilting the magnetic field you should be able to tilt the jet and steer.

It will take a while to get up to speed. I think - really it depends on the mass of the star and energy of the jet. Maybe not that long.

Brakes are less tricky than they seem: either swing around 180 degrees and point your thruster in the direction of your travel (hopefully not your mom's house), or open back up the front jet and muzzle down the back one. Or collision with a sufficiently massive object. You may want to disembark first.

$\begingroup$Funny idea. But turning off a astrophysical jet or even landing on the neutron star is definitely not hard science. (or at least, would require handwavium engineering)$\endgroup$
– b.LorenzFeb 4 '17 at 20:51

$\begingroup$Maybe a little one? Like the size of a bus?$\endgroup$
– WillkFeb 5 '17 at 21:04

In short YES, a solar sail is a viable method, the main current hurdle is scalability, which given futuristic materials and manufacturing techniques is easily overcome. Presently interstellar journeys are being mooted, rather than intergalactic as even at 1.0 C intergalactic journeys are in the order of millions of years, The technology is already in use today, and several white papers are available for further reading.

This older NASA white paper outlines solar sails as feasible, with the greatest short term risk being electrical charging, which is only really considered a risk in a heliocentric orbit. The greatest long term challenge as already mentioned is scaling due to manufacturing challenges. [1]

There is a (fantastic) news article about an ongoing mission; IKAROS, which has broached new ground, both achieving acceleration, and steering controlled by increasing and decreasing the diffusiveness of LCD panels located at the edge of the sail. [2]

This paper accurately and reasonably describes a (laser powered) round trip of interstellar length, taking 41 years, don't let the laser put you off, there are plenty of astronomical phenomena putting out significantly more than the 26TW laser described herein [3]

PROBE. - That's right - no living tissue to get screwed up by radiation - BUT somewhat delicate electronics. - This requires some thought about shielding

Propulsion mechanism = Solar Sail The single example we have done as a species was the IKAROS project - which the article seems to say got up to 400m/s

Shielding I would think pretty straightforward to implement adequate shielding to protect electronics. I would choose Z Layered shielding as described in the article. I would probably have the outermost layer be polished gold - kind of because that'd look COOL! But partly also because it's a pretty good shield - a little more effective than lead which is the most common material.

Bearing these things in mind I would probably go with a sail 100mtrs on a side and insert it into the radiation stream some distance from the Neutron Star - remember - the force is still going to drop off significantly as you move further from the source of the beam.

I think anything you write based on this concept - is going to have WAY bigger energy than what our sun is pumping out so you can do your calculations quite differently.

AT the end of the day I would think that yes - if you have one of those puppies pointed in the direction you want to send a probe - it'd be realistic to use it as a power source for a solar sail project.

HOWEVER Intergalactic distances being what they are - you are STILL not going to get any kind of quick turnaround - even if you get close to relativistic speeds - some kind of C Fractional velocity is STILL slower than light speed. Our nearest galaxy is Andromeda and that is 2.5Million light years away. Lets just say the intergalactic distance you want to travel is 3million light years - and you manage to eventually get your probe up to .9c before the acceleration effect wears off completely - you are probably still talking greater than 4 million years one way.

I can't see this being all that useful to be honest? I'm just not sure what any society would try and do some thing like that for?

If you want to do something realistic in a plot line I reckon you'd have to do so in some form of FTL. There's just no other meaningful way to cross intergalactic distances... Of course, that's just not a real possibility for us yet. I would personally try to utilise some concept around the same star/ particle beam to drive/ provide energy for a stable artificial wormhole. That could give you some kind of reasonable turnaround time for getting to the other end. Of course - that is in the realm of hand wavium... ;)